As a consequence of the constantly increasing integration density in microelectronics, photolithographic masks have to image structure elements that are becoming ever smaller into a photoresist layer of a wafer. In order to meet these requirements, the exposure wavelength is being shifted to ever shorter wavelengths. At the present time, argon fluoride (ArF) excimer lasers are principally used for exposure purposes, these lasers emitting at a wavelength of 193 nm. Intensive work is being done in regard to light sources which emit in the extreme ultraviolet (EUV) wavelength range (10 nm to 15 nm), and corresponding EUV masks. In order to increase the resolution capability of wafer exposure processes, a number of variants of the conventional binary photolithographic masks have been developed simultaneously. Examples thereof are phase masks or phase shifting masks and masks for multiple exposure.
On account of the ever decreasing dimensions of the structure elements, photolithographic masks, photomasks or simply masks cannot always be produced without defects that are printable or visible on a wafer. Owing to the costly production of photomasks, defective photomasks, whenever possible, are repaired. Two important groups of defects of photolithographic masks are, firstly, dark defects. These are locations at which absorber or phase shifting material is present, but which should be free of this material. These defects are repaired by removing the excess material preferably with the aid of a local etching process.
Secondly, there are so-called clear defects. These are defects on the photomask which, upon optical exposure in a wafer stepper or wafer scanner, have a greater light transmissivity than an identical defect-free reference position. In mask repair processes, these defects can be eliminated by depositing a material having suitable optical properties. Ideally, the optical properties of the material used for the repair should correspond to those of the absorber or phase shifting material. The layer thickness of the repaired location can then be adapted to the dimensions of the layer of the surrounding absorber or phase shifting material.
The material deposited for the repair should satisfy at least two further requirements. Firstly, it should withstand a predefined number of mask cleaning cycles substantially without alterations to its constitution, i.e. the optical properties and size. Secondly, a given number of wafer exposures should be able to be carried out with the deposited material, without the stated properties of the deposited material experiencing a significant change with regard to the adjacent absorber or phase shifting material.
WO 2012/146647 A1 describes the deposition of a reference marking on a photomask with the aid of a particle beam, a process gas and an additional gas, which may be an oxidizing gas.
WO 2005/017949 A2 describes the deposition of material on a photomask by use of an electron beam and TEOS (tetraethyl orthosilicate) or an organic or inorganic precursor gas.
The US patent having the number U.S. Pat. No. 7,727,682 B2 describes the repair of a phase shifting layer with the aid of an electron beam and the deposition gas TEOS. In order to protect the repaired location, in a second process step, a chromium protective layer is deposited on the phase shifting photomask over the whole area with the aid once again of an electron beam and of chromium hexacarbonyl.
The applicant has established that repaired locations of clear defects may be subject to a change in the course of the use of repaired photomasks. The document cited last reveals that a location repaired by the deposition of a suitable material has to be provided with a protective layer.
However, applying a whole-area protective layer on a repaired mask in order to cover one or more repaired locations is a time- and cost-intensive additional process step. Moreover, depositing this additional layer entails the risk that a layer that is not perfectly uniform will produce a new defect of the repaired photomask.
The present invention therefore addresses the problem of specifying a method and a device which enable a permanent repair of defects of absent material of a photolithographic mask and avoid at least some of the disadvantages discussed above.